Hafnium alloys



United States Patent US. Cl. 75-134 7 Claims ABSTRACT OF THE DISCLOSUREHafnium-base alloys suitable for nuclear reactor control rods consistof, by weight, at least one of the betastabilising elements zirconium30% and niobium 15- 30% up to 2% in total amount of at least one of thebetastabilising elements chromium, molybdenum, nickel and iron, up to0.5% silicon, and up to 2% in total amount of one or both of the alphastabilising elements tin and aluminum, the total content of alloyingelements being not greater than 30%, balance hafnium and impurities.

This invention relates to hafnium alloys.

Hafnium is used for control rods in nuclear reactors because of its highneutron absorption coefficient. However, there are occasions when amaterial with a lower coefficient but having a comparable or betterstrength and corrosion resistance is required.

According to the present invention, a hafnium-base alloy consists of, byweight, one or both of the beta stabilising elements, zirconium 530% andniobium 15 30%, 02% in total of one or more of the beta stabilisingelements, chromium, molybdenum, nickel and iron, 0-0.5% silicon, 0-2% intotal of one or both of the alpha stabilising elements, tin andaluminium, the total content of alloying elements being not greater than30%, balance hafnium and impurities.

The term impurities as used herein excludes zirconium which isfrequently present in hafnium up to about 4%.

The preferred ranges of composition are 5-25% zirconium, 15-25% niobium,0.1-2% in total of one or Patented June 2, 1970 ice 0.5% silicon, 0.1-2%in total of one or both of tin and aluminium. Preferably, the totalcontent of alloying elements should be between 10 and 30%.

The effect of the beta stabilising elements is to strengthen hafnium;the alpha stabilising elements, whilst having little effect on strengthwhen added alone, generally improve the properties -of the betastabilised alloy.

The addition of niobium and zirconium reduces the neutron absorptioncoefficient and improves tensile strength. Niobium is considerably moreeffective in this latter respect than zirconium when only one of theseelements is present. When both are present, the tensile strength is morethan double that of commercially pure hafnium whilst the ductility isreduced by only a small amount. Additions of small amounts of the otherbeta stabilising elements, chromium, molybdenum, nickel, iron andsilicon and of the alpha stabilising elements, tin and aluminium,further improve the tensile properties.

Corrosion resistance depends upon the alloying ele ments added and theamounts present. Zirconium has little effect on the excellent corrosionresistance of hafnium but niobium tends to increase the amount ofsurface film formed on exposure to high temperature pressurised steam.Below 15% niobium there is an appreciable loss of corrosion resistance.

The corrosion resistance of the alloys of the invention was determinedby exposing cleaned bright samples to steam at a pressure of 100atmospheres and at a temperature of 400 C. for 72 hours. The corrosionbehaviour was compared by the appearance of the surface of the samplesin accordance with the following classification.

(A) Glossy, bright, thin, adherent film (including interference films).

(B) Glossy, bright, thick, adherent film.

(C) Mottled appearance with dull patches or streaks which are usuallypowdery.

(D) Dull but adherent film.

It is important that the film should be adherent with no tendency topowder or to flake off. In the above classification, class (A) has thebest corrosion resistance.

The tensile properties and corrosion resistance of a number of alloys inaccordance with the invention are shown by way of example in the tablein comparison with pure hafnium. In the table, the abbreviation 4 /A,used in connection with the elongation values, refers to the gaugelength of test pieces, that is, a gauge length equal to four times thesquare root of the area of the crosssection of the test piece. Thecorrosion resistance was demore of chromium, molybdenum, nickel andiron, 0.01- termined in accordance with the above classification.

TABLE Elongation,

Composition Corrosion 0.2% P.S.. U.T.S., percent Alloy (analysed)resistance t.s.i. t.s.i. on 4 v2 Unalloyed T3 25Zr-0.5Mo A 15.7 30.6 23

We claim:

1. A hafnium-base alloy consisting of, by Weight, at least one betastabilising element selected from the group consisting of zirconium5-30% and niobium 15-30%; up to 2% in total of at least one betastabilising element selected from the group consisting of chromium,molybdenum, nickel and iron; up to 0.5% silicon; up to 2% in total of atleast one alpha stabilising element selected from the group consistingof tin and aluminium, the total content of alloying elements being notgreater than 30%; balance hafnium and impurities.

2. A hafnium-base alloy according to claim 1 in which the total contentof alloying elements is between 10 and 30%.

3. A hafnium-base alloy according to claim 1 containing 525% zirconium.

4. A hafnium-base alloy according to claim 1 containing 15-25% niobium.

5. A hafnium-base alloy according to claim 1 containing (ll-2% in totalof one or more of chromium, molybdenum, nickel and iron.

6. A hafnium-base alloy according to claim 1 containing 0.12% in totalof one or both of tin and aluminium.

7. A hafnium-base alloy according to claim 1 containing 0.010.5%silicon.

References Cited UNITED STATES PATENTS 2,234,969 3/1941 Hensel et al.75134 2,810,640 10/1957 Bolkcom et a1 75l34 RICHARD O. DEAN, PrimaryExaminer

